1. Field of the Invention
The present invention relates to a discharge lamp device for use in a vehicular headlight arrangement for an automobile, an electric railcar, a ship, an aircraft or the like, or in a general lighting system. The discharge lamp device includes a control circuit for controlling an electric power applied to a discharge lamp, an optical cable coupled at its one end to a light unit with the discharge lamp therein via a coupling arrangement, and a light distribution unit coupled to the other end of the optical cable for achieving a given light distribution.
2. Description of the Prior Art
In recent years, a metal halide lamp (discharge lamp) has drawn attention as a light source of the vehicular headlight. In general, it takes time for the discharge lamp to stabilize its luminous flux from the lighting thereof. Thus, for using it as a light source of the vehicular headlight, it is necessary to shorten the time until the luminous flux is stabilized.
For achieving this, a method has been known as disclosed for example in Japanese First (unexamined) Patent Publication No. 6-54521, wherein an excessive lamp current is fed to the discharge lamp just after the lighting so as to shorten the time until the luminous flux is stabilized. Specifically, upon cold starting of the discharge lamp, the lamp current is controlled at a given constant current value greater than a stable-time current value for a given fixed time from the lighting, and then the lamp current is exponentially reduced using a timer circuit so that a stable lighting state is reached.
In this conventional method, however, as shown in FIG. 7, the luminous flux at the start of the lighting may be subjected to overshoot A or undershoot B relative to the luminous flux at the stable time, and further, a build-up characteristic D of the luminous flux may be subjected to dispersion due to dispersion in quality of the individual discharge lamps. Thus, the foregoing conventional method is not preferable for the vehicular headlight.
On the other hand, an optical cable is composed of a clad member and a core member having mutually different refractive indexes and used for transmitting the light by means of the total reflection at an interface between the clad member and the core member due to a difference in refractive index. In recent years, this type of optical cables has been used in various industrial fields, such as, optical communications, optical sensors, decorative displays and various lighting systems of automobiles, optical instruments, medical instruments, outdoor signboards, domestic electrical devices and so forth.
For example, Japanese First (unexamined) Patent Publication No. 61-250902 proposes a light irradiation arrangement, wherein light emitted from a light source is transmitted through an optical cable of the foregoing type to an object (to be irradiated) arranged at a given place such that radiation heat from the light source is not transferred to the object. The optical cable proposed in this publication is arranged such that a fluororesin plastic pipe having a small refractive index is filled therein with liquid-phase glycerin for the total reflection of the light, and glass rods are fitted into the plastic pipe at both opposite ends thereof for sealing.
However, in such an optical cable filled with the liquid such as glycerin, since the plastic pipe is sealed by the glass rods fitted in at the ends thereof, there is raised a problem of leakage of the liquid from between the plastic pipe and each of the glass rods.
On the other hand, another optical cable of the foregoing type has been developed and manufactured, wherein a clad member made of synthetic resin with a small refractive index and a core member made of synthetic resin with a refractive index greater than that of the clad member are formed integral with each other to form a flexible optical cable.
However, if such an optical cable of the synthetic resin is located near a light source, such as a halogen lamp or a high-pressure mercury lamp, there is raised a problem that the optical cable is softened or deformed due to the direct heat or the radiation heat from the light source so as to cause a dislocation of an optical axis. If the dislocation of the optical axis occurs, the utilization efficiency and the transmission efficiency of the light are deteriorated so that the object can not be lightened with the required illuminance.
On the other hand, if a glass rod having a heat cut filter function is disposed at an end portion, near the light source, of such a synthetic resin optical cable, the direct heat or the radiation heat from the light source can be intercepted by the glass rod. However, the light entering the glass rod from the light source leaks to the exterior through the peripheral walls of the glass rod so that the quantity of the incident light into the optical cable is reduced. Thus, the utilization efficiency and the transmission efficiency of the light are lowered so that the object can not be lightened with the required illuminance.
It may also be considered to integrate the optical cable and the glass rod using the clad member of the optical cable. However, this is not practical since the core member and the clad member are formed integral with each other in this type of the synthetic resin optical cable so that a process for removing the core member is required.
On the other hand, Japanese First (unexamined) Patent Publication No. 63-40201 proposes a vehicular headlight arrangement, wherein the desired spreading of a light distribution in left and right directions can be achieved without causing an upper boundary to be ambiguous. Specifically, in this publication, a light distribution pattern forming shade plate is disposed at approximate focal points of condenser lenses forming a condenser lens unit, and a reflecting mirror is provided for producing the reflected light which converges along a line connecting the focal points of the respective condenser lenses. With this arrangement, shading boundaries caused by the light distribution pattern forming shade plate are projected forward through the condenser lens unit so that the light distribution as shown in FIG. 20B can be achieved.
However, in such an arrangement, about a half of the reflected light from the reflecting mirror is intercepted due to the light distribution pattern forming shade plate as shown by hatched lines in FIG. 20A. Thus, there is raised a problem of poor utilization efficiency of the light and thus an occurrence of the insufficient illuminance. Further, for achieving the desired light distribution, the reflecting mirror, the light distribution pattern forming shade plate and the like are required so that the number of the components is increased to render a structure of the vehicular headlight arrangement complicated.